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The change of state from liquid to gas is known as vaporization.If this change of state takes place from the surface of the liquid only,then the process is known as evaporation.This evaporation can take place at any temperature.Evaporation of water means escape of water particles(molecules) from the surface as water vapour. Any molecule that happens to have a bit of extra energy and which is near the surface of any liquid will tend to move up and into the air never to return even at ordinary temperature.The higher the temperature,the more energy the molecules have and this increases the chance of more molecules have enough energy to leave the liquid. When boiling point is reached ,all the molecules attain enough energy to leave the liquid and they try to so at once.